1. Field of the Invention
The present invention relates to tempering glass sheets and, more specifically, to the tempering of glass sheets using quenching materials having a high heat transfer rate, such as a sublimable material such as carbon dioxide.
2. Description of the Technical Background
The present invention relates to the tempering of glass sheets, and particularly to the tempering of relatively thick glass sheets having a thickness of 1/4 inch and greater. The need to develop large tempering stresses in thick glass sheets requires the use of a quenching medium having a high capacity for heat exchange.
The present invention has utility especially for tempering glass sheets having a nominal thickness of 6 mm (1/4 inch) and greater for use in laminated and monolithic windows for automobiles, railroad, aircraft and marine vehicles. However, the principles of this invention are also applicable for use in tempering glass sheets of other thicknesses and it is also understood that other material such as metals, metal alloys, plastics and glass fibers may also be chilled according to the teachings of the multiple stage cooling process of the present invention.
Glass sheets are thermally tempered by heating them to an elevated temperature above the glass strain point approaching the glass softening point, and then suddenly chilling the glass to cool the glass surface regions relatively rapidly while the inner regions of the glass cool at a slower rate. The differential cooling throughout the glass thickness develops a compressive stress in the glass surface regions which is balanced by a tension stress in the interior of the glass. The resultant tempered glass has a much greater resistance to fracture than untempered glass. Furthermore, in the less frequent occurrences when tempered glass does fracture, its breakage pattern is significantly different from that of untempered glass. Tempered glass shatters into small fragments having blunt, rounded edges. The size of these fragments is smaller and their surfaces are smoother when they develop from more highly tempered glass that is fractured. Untempered glass, on the other hand, fractures to form relatively large pieces having sharp, jagged edges. This safer breakage pattern of tempered glass and its lesser tendency to fracture makes tempered glass more desirable for use in many articles, such as transparent doors, motor vehicle closures, ophthalmic lenses, covers for instrument panels, containers, and the like.
Traditionally, glass has been thermally tempered by impinging blasts of cool air on the surface of heated glass articles. This technique is limited for practical purposes to develop compressive stress of relatively low magnitude that can be developed with air because air has limited heat transfer capacity when it impinges with a force that does not degrade the surface quality of an impinged glass sheet. When air is blasted more rapidly to improve its heat transfer qualities, the blasts or sheets of air applied against the hot glass surface cause distortion in the glass surfaces. This distortion is termed "batter" by those skilled in the art.
It is desirable to use a quenching medium that has a higher heat transfer coefficient than air. Sublimable carbon dioxide stored as a liquid under pressure has been suggested as a means of tempering hot glass sheets in U.S. Pat. No. 3,764,403 of James E. Neely, Jr. This patent discloses the application of soft particles of a sublimable solid toward the surface of a glass sheet heated to an elevated temperature sufficient for tempering on sudden chilling to chill the glass surface at a sufficiently rapid rate to impose at least a partial temper thereto. The heat of sublimation of carbon dioxide has been an important element in promoting rapid cooling that makes possible the tempering of glass sheets as thin as 1.5 mm (0.060 inch) and less. In the past, the cost of sublimable carbon dioxide has discouraged its use.
U.S. Pat. No. 3,883,339 to Edmund R. Michalik and James E. Neely, Jr., discloses a more economical way of tempering hot glass sheets by sublimation tempering using relatively expensive soft particles of sublimable material for the initial cooling either in the form of carbon dioxide snow carried by carbon dioxide gas or carbon dioxide mixed with air blasts for an initial cooling step followed by the use of air or other tempering media after the temperature throughout the thickness of the article being tempered is cooled below the strain point of the glass.
The aforesaid patent also suggests the mixing of the soft sublimable particles of carbon dioxide with air blasts so that the soft sublimable particles are applied in a cooling medium containing air blasts. This mixture not only is more economical but also moderates the rate of cooling and improves the resistance of the glass to fracture during the tempering operation.
U.S. Pat. No. 3,929,442 to James E. Neely, Jr., discloses apparatus for tempering glass sheets using a soft sublimable cooling medium such as carbon dioxide snow. This patent specifies that the apertures through which the sublimable material is applied toward the glass surface occupy less than 5% of the area of each said surface and that the exhaust paths facing the glass sheet surfaces occupy at least 50% of the areal extent of each glass sheet surface undergoing quenching.
Belgium Pat. No. 729,055 proposes heating a glass sheet to a temperature as high as 760.degree. C. (1400.degree. F.) followed by a multiple stage cooling technique involving first cooling the heated glass sheet by air blasts to harden the surfaces, thereby cooling them to a temperature on the order of 560.degree. C. (1040.degree. F.) to 640.degree. C. (1184.degree. F.) and developing a temperature gradient of 60.degree. C. (108.degree. F.) to 120.degree. C. (216.degree. F.) from the center to each surface of the glass sheet followed by more rapid cooling by immersion in a liquid quenching medium. This method requires heating the glass to such an elevated temperature initially that the glass is penetrated by supporting tongs so deeply as to provide poor optics in the region of the tong gripping. Furthermore, in using a liquid quenching medium, it is necessary to immerse the glass sheet incrementally into the liquid quenching medium. This enhances the likelihood of breakage because of the stress patterns that are formed because of the development in the glass sheet of a steep temperature gradient in the direction of movement into the liquid tempering medium. Furthermore, in using a liquid quenching medium, it is necessary to remove the liquid before one can use the tempered glass. The removal of quenching liquid usually involves an expensive washing and drying operation.
French Pat. No. 2,024,397 to St. Gobain discloses a glass sheet tempering apparatus in which horizontally supported glass sheets are subjected to a pair of obliquely directed elongated, opposed air blasts while moving continuously past and between a pair of elongated slots through which oblique streams of cold air are directed across the entire width of the glass sheet to reduce iridescence and to support and convey the glass sheets towards the rest of the tempering apparatus followed by exposure of the moving glass to additional blasts of air. The exclusive use of air limits the maximum temper obtainable in the glass unless the air blasts are so strong as to develop optical distortion in the major surfaces of the hot glass sheets subjected to such air blasts.